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[84.226.167.205]) by smtp.gmail.com with ESMTPSA id t27sm9126163ejc.62.2021.03.16.03.11.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 Mar 2021 03:11:43 -0700 (PDT) Subject: Re: [PATCH] tty: serial: samsung_tty: remove spinlock flags in interrupt handlers To: Johan Hovold Cc: Greg Kroah-Hartman , Jiri Slaby , linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, marcan@marcan.st, arnd@kernel.org, Andy Shevchenko References: <20210315181212.113217-1-krzysztof.kozlowski@canonical.com> <7f348e4c-3051-13cf-d461-eeda0ef53fdd@canonical.com> From: Krzysztof Kozlowski Message-ID: <816834ba-8dc2-05cd-9c81-a11d65980cfd@canonical.com> Date: Tue, 16 Mar 2021 11:11:43 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/03/2021 10:56, Johan Hovold wrote: > On Tue, Mar 16, 2021 at 10:47:53AM +0100, Krzysztof Kozlowski wrote: >> On 16/03/2021 10:02, Johan Hovold wrote: >>> On Mon, Mar 15, 2021 at 07:12:12PM +0100, Krzysztof Kozlowski wrote: >>>> Since interrupt handler is called with disabled local interrupts, there >>>> is no need to use the spinlock primitives disabling interrupts as well. >>> >>> This isn't generally true due to "threadirqs" and that can lead to >>> deadlocks if the console code is called from hard irq context. >>> >>> Now, this is *not* the case for this particular driver since it doesn't >>> even bother to take the port lock in console_write(). That should >>> probably be fixed instead. >>> >>> See https://lore.kernel.org/r/X7kviiRwuxvPxC8O@localhost. >> >> Thanks for the link, quite interesting! For one type of device we have >> two interrupts (RX and TX) so I guess it's a valid point/risk. However >> let me try to understand it more. >> >> Assuming we had only one interrupt line, how this interrupt handler with >> threadirqs could be called from hardirq context? > > No, it's console_write() which can end up being called in hard irq > context and if that path takes the port lock after the now threaded > interrupt handler has been preempted you have a deadlock. Thanks, I understand now. I see three patterns shared by serial drivers: 1. Do not take the lock in console_write() handler, 2. Take the lock like: if (port->sysrq) locked = 0; else if (oops_in_progress) locked = spin_trylock_irqsave(&port->lock, flags); else spin_lock_irqsave(&port->lock, flags) 3. Take the lock like above but preceded with local_irq_save(). It seems the choice of pattern depends which driver was used as a base. Best regards, Krzysztof